Framebuilders & techies – What rear post mount offset?

I’m just checking my details before sending off the final CAD models and Drawings for our BMX race bike (URP MoTo if you’re interested) for manufacture and I want to check the (rear) Post mount offsets from the face of the dropout so that it has maximum compatibility.

I’ve currently got it at 5.7mm as per this Shimano drawing –

But it seems to be at the limit of what a Hope caliper will allow without widening the caliper slot. So I just want to check there’s not some universal one size fits all dimension I’m missing.

I’ve got a very contradictory drawing from Hope that suggests the Shimano dimension is 4mm and their’s is 5.25 but that doesn’t tie up with the 5.7 shown above. I is a little confused.

Is there some overarching Hayes standard that I can’t find being as they invented? the postmount std.

Cheers in advance

Gary

You will find this varies I believe when speaking with Owen they use what you could loosely term an industry standard from the inside of the dropout to the disc

I get all manner of shite through the door that has been welded wrong let alone follows a 1.25 mm tolerance which believe it or not is just short of the full adjustment range of a calliper one way

This is the problem with post mount on the original ISO to post mount you could shim the ISO over the error distance , now you have to get it right

Thanks Mike – fortunately the design means there’s no tolerance issues (other than machining) as the dropout/caliper are all in one piece. I’m just trying to do some kind of catch all that accommodates any of the individual manufacturer’s “standards”* if that’s at all possible.

* ****ing standards!

here are some pictures

Untitled by bicycle manufacturing[/url], on Flickr

Untitled by bicycle manufacturing[/url], on Flickr

remember when you have a bolt in at 5.85mm (m6) then theres 2mm a side minus a gnats nadger to play with and then shimano you lose 0.5mm a side because their slot is narrower still than hope so your down to 1.5mm adjustment which when you consider

I v’e had frames on the fixtures here that are nearly 1/4″ out of alignment from headtube to dropout …. ..and you figure your out of tolerance

In the end its a case of modelling it all super accurate and seeing what you have to play with if its 0.25 mm its not a lot

You’d think with 4mm of adjustment on the slot you’d never get issues but . . . . 🙄

You only get issues if you try to use hope stuff with other brands. Hope offset has always been squiffy compared to shimano and everyone else.

So does everyone have issues fitting Hope stuff directly to frames? ie without an adapter?

As a total aside I’ve seen that design before and wondered why you were going with belt drive with the associated efficiency losses compared with a chain?

Steve

We aren’t necessarily advocates of the belt drive because of it’s efficiency – if ultimate efficiency is a concern you’ll be running lightly oiled ceramic bearings everywhere loosing all the bearing seals and fitting a clean drivetrain every day/race. Where the belt really scores is weight – it’s an instant 150g loss over an equivalent BMX sized chain drive, silence – you only realise how noisy a chain is when you get on one of these and general snap – there’s just no slop in it, again something you don’t notice until you swap from one to the other.

The design isn’t belt specific – it works just as well with a chain but it certainly accommodates a belt without any compromises and it is without doubt the best solution to fitting a disc to a BMX yet devised (imho). Pain in the arse to make though and therefore a bit more expensive – for now. ;o)

Oh and it’s a Watt difference at 250W load according to THIS report and that’s comparing to a fresh clean chain drive and who has one of those? And BMXers kick significantly more than 250W so one here or there may make little difference.

it’s an instant 150g loss over an equivalent BMX sized chain drive

Even with the belt drive sprockets?
Don’t get me wrong I’m not outrightly criticising, I’m just curious as to how you arrived at belt drive on a race BMX.
The snap thing might just swing it I guess.

Here’s sime pics off a post on the URP FB page showing the comparison –

Chain VS Belt

Chain inc rngs

Belt inc cogs

Ok that’s with a not so light chain but then the rear sproket is alloy so it’s lighter than it’s less durable by comparison than the SS Gates one. So yes it’s significant.

BUT the biggest thing is the silence – it’s just so much less rattly than a chain setup – I think the belt is self setting to a degree – because it wants to straighten out to a degree, it doesn’t have the flop a chain does between the cogs. You’d have to try it to get what I mean.